Separation device for microparticles involving a magnetic rod

ABSTRACT

The invention relates to a means for separating magnetic particles from a composition. The means comprises an elongated protective cover ( 1 ) that comprises a recess ( 2 ) extending from the upper end towards the lower end thereof, the recess comprising a movable magnetic rod ( 4 ), the proportion of the length of the rod to its thickness being at least about 2:1. The invention can be used in different applications especially in the fields of biotechnology, biochemistry, and biomedicine. Collecting particles by using the means is easy and fast.

TECHNICAL FIELD

The invention relates to the separation of magnetic particles from acomposition. The invention can be used in different applications,especially in the fields of biotechnology, biochemistry, andbiomedicine.

BACKGROUND OF THE INVENTION

Magnetic microparticles are used as a solid phase in variousapplications to bind biomaterial. One advantage of microparticles is thelarge area of the solid phase and short diffusion lengths. The size ofmicroparticles is generally 0.05-10 μm and they are available indifferent materials and already activated for many applications.Magnetic particles can be moved by using a magnet.

The separation methods of magnetic particles currently used includesettling a reaction vessel in a magnetic field so that particles areaccumulated into a so-called pellet at the bottom of the vessel.Thereafter, the liquid which is free from particles is decanted orremoved by aspiration. However, the removing of the liquid from thevessel must be carried out very carefully so as not to remove theparticles at the same time.

Publication WO-86/06493 proposes a method to be used in immunoassays, inwhich magnetic particles and the marked complex adhered to them areseparated from a liquid by using a magnetic rod and subsequently takento be measured. The tip of the rod comprises a fixed magnet and aremovable protective cover to whose outer surface the particles adhere.It is preferable to cover the protective cover with another cover afterthe separation and before measuring. After the measurement, theprotective covers are detached together with the particles and thrownaway and new covers are taken for a new separation. According to thepublication, the magnet can also be an electromagnet, whereby themagnetic field can be eliminated when desired.

Publication WO-87/05536 proposes a device for separating magneticparticles, comprising, on the inside, a rod movable in a vertical boringand a magnet at the lower end thereof. The device is introduced, withthe magnet in the lower position, into a liquid containing particles,whereby the particles are accumulated on the end of the rod. When themagnet is allowed into the upper position, the particles can be detachedfrom the rod. In this way, particles can be collected and transferredfrom one liquid into another.

However, the disclosed separation devices and methods for magneticparticles cannot be applied very well in applications in which particlesmust be collected from a fairly large volume and transferred into anessentially smaller one.

SUMMARY OF THE INVENTION

Now, a separation method according to claim 1 has been invented. Theother claims present some preferred embodiments of the invention.

The device according to the invention comprises an elongated protectivecover that includes a movable rod comprising one rod magnet in thelongitudinal direction of the cover. The proportion of the length of therod magnet to its thickness is at least about 2:1, preferably at leastabout 3:1, and most preferably at least about 12:1. Both the intensityand the gradient of the magnetic field thus formed are the strongest atthe end of the rod and when the magnet is in the lower position, theparticles from the composition are accumulated directly on the tip ofthe cover. Particles can be released from the tip of the cover into avolume which is many times smaller than the original composition.

The rod magnet preferably consists of a permanent magnet and aferromagnetic arm which is its extension.

The rod magnet is preferably sufficiently long so that the upper end ofits dipole always remains above the surface of the composition. Ifparticles are to be collected from a composition column higher than thedipole, it must be seen to that the particles from the upper part of thecolumn are first collected on the tip so that the upper end of thedipole is constantly above the particles.

Since the upper end of the magnet can be above the composition, a moreeffective magnet with respect to the volume of the composition can beused, accelerating and facilitating the collection.

The device according to the invention comprises, in the tip of theprotective cover, an intensive magnetic field in the direction of thecover. This is especially advantageous when particles are collected froma concentration of particles which has first been created in some otherway. An additional advantage is that a high retaining strength can beprovided directly in the tip from which the adhesion of the liquidparticularly tends to detach particles when the protective cover islifted off the liquid.

The tip of the protective cover is preferably provided with a sharpdownward projection. This minimises the amount of liquid remaining inthe tip. Typically, the tip is shaped like a cone. When transferringparticles into very small vessels, the tip is preferably shaped as acone with a concave surface.

When a ferromagnetic arm is used in the magnet, the magnet and themagnetised arm together function as a long rod magnet. The arm dissolvesthe gradient of the upper pole of the field, whereby the upper pole doesnot carry out the collection of particles. In this way, the long rodmagnet can be provided at a low cost. However, even with a ferromagneticarm, it is advantageous to use a relatively long magnet (with a lengthof about 1.5 . . . 10 times the thickness). The length of the magnet ispreferably selected so that a maximum internal, permanent fieldintensity is provided for the magnet in question.

The junction between the magnet and its arm is preferably made so thatthe arm and the magnet come inside one another for a short length. Inthis way, the formation of strong gradients at the junction, which maypossibly collect particles, is avoided.

The cross-section of the rod magnet can be, e.g., circular orrectangular. The circular shape is the best with respect to bothmanufacture and use. Indeed, the rotation of the magnet on its axis, forexample, has no effect in this case. In principle, the rod can be curvedto make the moving mechanisms simpler.

The shape of the protective cover on the rod can vary according to theuse. Normally, the circular shape is the most advantageous with respectto both manufacture and use. In order to increase strength, the covercan be made conical, which also facilitates the manufacture of the coverby injection moulding. The cover is preferably made of polypropylene.

The invention is best-adapted to be used for particles of about 1-10μm.

BRIEF DESCRIPTION OF DRAWINGS

Some preferred applications of the invention are described in thefollowing as examples. In the drawings of the description:

FIG. 1 presents a separation means according to the invention,

FIG. 2 presents the use of the means of FIG. 1 for collecting particlesfrom a suspension,

FIG. 3 presents the use of the means of FIG. 1 to release the collectedparticles into a very small vessel,

FIG. 4 presents the use of another means according to the invention forcollecting particles from the wall of a test tube,

FIG. 5 presents the use of a third means according to the invention forcollecting particles from a small amount of liquid; and

FIG. 6 presents the use of the means of FIG. 5 for releasing thecollected particles into a small amount of liquid.

DETAILED DESCRIPTION OF THE INVENTION

The separation rod according to FIG. 1 comprises an elongated protectivecover 1 and a bore 2 in it. The lower ends of the cover and the bore areslightly tapered. The upper end of the body is provided with flange 3which facilitates gripping.

Bore 2 comprises loose magnetic rod 4. This comprises vertical rodmagnet 5 at the lower end thereof, and ferromagnetic arm 6 above it asan extension of the magnet. Gripping knob 7 is provided at the end ofthe arm.

The lower end of the cover is provided with a tapering, sharp-edged tip8 with a concave surface. The length of the tip correspondsapproximately to the width of the lower end of the cover.

FIG. 2 presents the collection of particles from a test tube where theyare dispersed in suspension 9. The separation rod is moved in the testtube by sweeps up and down. In this way, particles can be collected atthe tip to form an annular mass 10. When magnet 5 is kept in the lowerend of bore 2, the particles remain attached to the tip. When particlesare to be released, the magnet is lifted up.

Tip 8 is especially well-adapted to transfer particles into a very smallvessel, such as the well of a so-called HLA plate 11 (FIG. 3). The tipis slightly longer than the height of the well. When the tip is pushedinto the well, the surface of the liquid rises along the surface of thetip due to surface tension. The edge of the moving surface of liquidsweeps the particles off the tip and into the liquid. The detachment canbe improved by stirring the rod. Correspondingly, when the tip is liftedfrom the well, the surface of the liquid moves towards the sharp end ofthe tip as an integral film in this way, the liquid and the particleswith it are completely detached from the tip.

The particles are preferably detached from the liquid so that they arefirst concentrated into one spot in the vessel from where they are thencollected by using the rod. The concentration can be effected by lettingthe particles settle by gravitation, by centrifuging or by pulling theparticles onto the wall of the vessel by using a magnetic field.

FIG. 4 presents the collection of particles from the wall of a test tubeon which they were first pulled, by using a magnet, to form a verticalstrip 9. By sweeping along the strip by the tip of the rod, theparticles can be made to adhere to the tip of the protective cover 1 ofthe rod to form mass 10. Here the tip of protective cover 1 is shapedlike a relatively blunt cone. This is well-adapted for transferringparticles into vessels where the tip can be pushed properly into theliquid.

The proportion of the length of magnet 5 to the diameter is about 10:1,and the proportion of the length of the arm to the length of the magnetis about 5:1. The arm is slightly wider than the magnet and the upperend of the magnet is embedded inside the lower end of the arm at alength of about twice its diameter.

FIGS. 5 and 6 present a separation device including a magnetic rod 4′which comprises only a permanent magnet 5′ coated with a suitableprotective layer but no ferromagnetic arm. The upper end of protectivecover 1′ is provided with a gripping portion 3′ with two flanges whichenables the easy control of the device in automatic separation oranalysis devices, for instance. The proportion of the length of themagnet to the diameter is about 7:1. The tip of the cover is shaped likea relatively blunt cone (the height of the cone being about ⅓ of itswidth).

FIG. 5 presents the separation of particles from a relatively narrowvessel compared with the separation rod (the inner diameter of thevessel is about 70% of the diameter of the rod) and from a relativelysmall amount of liquid. Lower line 11 presents the free surface ofliquid and upper line 12 the surface of liquid when the separation meansis pushed into the liquid. Magnetic particles are accumulated at the tipof the protective cover to form mass 10 which can be released back intoa relatively small amount of liquid (FIG. 6).

The means of FIG. 5 and 6 is well-adapted to be used in the wells ofmicrotitration plates with diameters of about 7 mm.

What is claimed is:
 1. A means for separating magnetic particles from acomposition, comprising: (a) a separation rod comprising a rod magnetincluding an elongated magnet at a lower end and a ferromagnetic armattached to the elongated magnet; and (b) an elongated protective coverwith a tip, wherein the resulting magnetic field is strongest at thelower end of the separation rod for accumulation of the particles fromthe composition on the tip of the protective cover, and wherein theprotective cover covers the separation rod and is movable with respectto the rod, for release of the particles from the means for separating.2. The means according to claim 1, wherein the proportion of the lengthof the rod magnet to its thickness is at least about 2:1.
 3. The meansaccording to claim 2, wherein the proportion of the length of the rodmagnet to its thickness is at least about 3:1.
 4. The means according toclaim 2, wherein the proportion of the length of the rod magnet to itsthickness is at least about 12:1.
 5. The means according to claim 2,wherein the proportion of the length of the elongated magnet to itsthickness is at least about 10:1 and the proportion of the length of theferromagnetic arm to the length of the elongated magnet is about 5:1. 6.The means according to claim 2, wherein the ferromagnetic arm is thickerthan the magnet and the upper end of the magnet is embedded inside thelower end of the ferromagnetic arm at a length of about twice thediameter of the magnet.
 7. The means according to claim 2, wherein thetip of the protective cover is cone-shaped with a concave surface.
 8. Amethod for separating magnetic particles from a liquid composition in avessel, comprising: (a) providing a means for separating the magneticparticles, comprising: (i) a separation rod comprising a rod magnetincluding an elongated magnet at a lower end and a ferromagnetic armattached to the elongated magnet; and (ii) an elongated protective coverwith a tip, wherein the resulting magnetic field is strongest at thelower end of the separation rod, and wherein the protective cover coversthe separation rod and is movable with respect to the rod; (b) placingthe means for separating in the vessel containing the composition toaccumulate the particles from the composition on the tip of theprotective cover; and (c) removing the means for separating, along withthe particles, from the vessel.
 9. The method according to claim 8,wherein the means for separating is placed into the vessel such that anupper end of the long magnet remains above a surface of the liquidcomposition.
 10. The method according to claim 8, further comprisingreleasing the particles from the means for separating into a furthervessel by moving the protective cover with respect to the separationrod.
 11. The method according to claim 8, wherein the proportion of thelength of the elongated magnet to its thickness is at least about 2:1.12. The method according to claim 11, wherein the proportion of thelength of the elongated magnet to its thickness is at least about 3:1.13. The method according to claim 12, wherein the proportion of thelength of the elongated magnet to its thickness is at least 12:1.